Referring to FIG. 1, a conventional single-phase high-power-factor switchable power supply 1000 which comes with two levels of circuit framework. The front level is a non-insulated PFC rectifier 1100, and the rear level is an insulated DC-DC converter 1200. FIG. 2 and FIG. 3 depict conventional switchable power supplies 2000, 3000, respectively. FIG. 2 shows that a PFC boost AC-DC rectifier 2100 is coupled to a full-bridge phase shift DC-DC converter 2200. FIG. 3 shows that a boost AC-DC rectifier 3100 is coupled to a LLC resonant DC-DC converter 3200. The switchable power supplies 2000, 3000 of FIG. 2 and FIG. 3 are applicable to AC power SAC and loads.
However, conventional PFC circuits have drawbacks as follows: (1) switchable power supplies which come with a bridge rectifier inevitably incur high conduction loss; (2) switchable power supplies which dispense with a bridge rectifier have a circuit framework lacking any transformer; (3) due to high capacitance on the DC output side of a boost PFC converter, AC voltage input instantaneously charges the DC capacitor and thus generates a high input current, causing failures of the rectifier or the output diode, not to mention that the DC capacitor is likely to be overcharged and thus causes damage to main power device; and (4) in a control system for the switchable power supplies, power factor-related control is exercised on the primary side, whereas the DC-DC converter is place under control on the secondary side; hence, it is necessary to achieve separation of the primary and secondary sides in order to enable the control system to operate smoothly, thereby rendering the circuit intricate.
Accordingly, it is imperative to provide an insulated PFC circuit framework which overcomes the aforesaid drawbacks of the conventional PFC circuit.